To mechanically refine pulp in an effective way to e.g. produce paper is a non-trivial subject. A plethora of refiners and refiner elements have been designed in order to further improve the mechanical refining action on the pulp.
A commonly used pulp refiner comprises a rotor unit and a stator unit that are aligned along a pulp feeding axis facing each other. The refining of the pulp is performed in a bounded area between the rotor unit and the stator unit. During use of the pulp refiner pulp is fed via a feeding channel through a hole in the stator unit to emerge in an area bounded by the stator unit and the rotor unit. The rotor unit facing the stator unit is arranged on a rotatable axis that can be rotated by means of an electrical motor. The purpose of the rotor unit, which in the following will be simply referred to as a rotor, is to grind the pulp between a surface of the stator unit and a surface of the rotor. Thus, when pulp leaves the feeding channel and enters the bounded area between the rotor and the stator through a hole provided in the stator it flows in on the rotor and due to the rotation of said rotor the pulp is directed outwards towards the boundaries of the rotor and stator. On the boundaries there are usually provided refining segments on the surfaces of the rotor and/or the stator. The purpose of these refining segments is to improve the grinding action on the pulp.
One problem with such designs is that the pulp will be directed towards the boundaries in an uneven fashion. Large chunks of pulp will be localized in a particular position on the rim of the rotor/stator arrangement while other positions will be more or less devoid of pulp. This will in turn lead to uneven grinding of the pulp. Thus efforts have to be made to improve the distribution of the pulp.
Another problem within the art is that part of the pulp initially can get stuck in the middle of the center plate. This might lead to pulp piling up in the middle of the center plate which can negatively affect the pulp distribution. It is therefore also a need for a center plate design that opposes that pulp gets piled up in the middle of the center plate.
A known measure to achieve such a pulp distribution is to provide the center plate of the rotor surface with a set of wings or wing profiles, whose purpose is to direct the pulp more evenly towards the rim of the stator/rotor arrangement. These wings are protrusions provided on the surface of the rotor facing the incoming pulp. The wings are mostly curved to obtain an arc-shaped form. By means of such wings pulp will be directed into the open channels defined between adjacent wings to thereby give a more even distribution of the pulp in the refining area. The constraints regarding the design of the wings constitute a very delicate dynamic problem where such phenomena as flow turbulence of the pulp have to be taken into consideration. This makes mathematical modeling of the dynamics intractable and direct observation of the wings effect on the dynamics is also complicated, partly due to the smallness of the refining area.
Several attempts have been made to improve the distribution of pulp. In U.S. Pat. No. 3,902,673 there is disclosed a refining machine for fragmented beating material. The machine comprises a rotationally symmetric annular infeed channel formed between a central body and an outer part surrounding the central body. The infeed channel has an increasing inner and outer diameter in the material feeding direction. The infeed channel redirects the movement of the material essentially radially outwardly in relation to the center of the rotating means. In U.S. Pat. No. 6,206,309 B1 there is disclosed an apparatus for refining lignocellulosic material between two relatively rotating elements. The rotary element is provided with a central feeding means comprising an axial screw and wings. In U.S. Pat. No. 4,396,161 there is disclosed a disk refiner for cellulose, paper or equivalent pulp provided with guiding vanes. The vanes are provided on the stator side of the refiner.
None of these proposed technologies are however optimal, hence there is a continued need in the art to further improve the pulp distribution for a pulp refiner. There is also a need to improve the distribution of the pulp in a way that softens the problems related to pulp getting stuck and piled up in the middle of the center plate.